We must be able to understand the population dynamics of parasites in order to improve control strategies, but studying this has ethical complications in human systems. Studying animal (reservoir) hosts can help alleviate this problem

Theron et al. Studied Rattus Rattus reservoir hosts from 1983 to 1990 The island of Guadeloupe A variety of study sites were used in an environment changing from marshy forests, through cattle graze land into mildly and then heavier human populated areas. All study sites flood and dry out seasonally Captured rats during October and November each year Worms were removed from rats by hepatic perfusion and careful examination of the liver and lungs Snails were sampled using scoops mounted on wooden handles and sampled for cercarial shedding, although infection dynamics in the snails were not analysed

 Prevalence: The percentage of the host population that is infected  Mean Intensity: The number of parasites per infected host  Mean Abundance: The number of parasites per host for the entire host population  Over Dispersion: When there is greater variability in the data set than the predicted results

The removal of rats for examination had no effect on the rat population dynamics - Trapping efficiency had low annual variation

There was a strongly clumped worm distribution with strong spatial heterogeneity of parasite transmission 50% of the parasites can be found in 4% of the available hosts 9% of the infected hosts harbour ½ of the parasite population

The over-dispersion shown in Fig. 3 can be explained through things such as genetic, ecological or behavioural aspects of the parasite. The patchiness of habitats that include standing water, mixed with a low infection rate of snails (25%) limits the dispersal ability of the cercariae Only three of the eight study sites showed infected snails This focuses the infection of rats to isolated areas, leading to unequal infection of hosts Territoriality of rats, along with high population densities reduces host movement, further reducing parasite transmission.

For the whole population, variance in abundance was low and infection levels remained at an equilibrium Local populations were much more variable and were in a patchy environments characteristic non- equilibrium state In strongly clumped distributions, very low variation in prevalence may induce a very high variation in abundance A drought hit the region in 1983 and 1984, which can be considered the reasoning for the overall low abundance during 1984 and 1985

 A genetic divergence in cercarial emergence timing  Some population sites had peak emergence in the late morning while others had peak emergence closer to late afternoon  Those that emerged later in the day were more likely to encounter, and therefore infect their rat host

Human participation, due to treatment for infection, in parasite transmission has been decreasing Selection pressures on Schistosoma by their rat hosts can create micro- evolutionary adaptations in the local populations of the worm This leads to an increased risk of extinction of these local populations, especially those which have not adapted to the life style of their rat host The populations will be under stronger selection pressure for any traits, such as later cercariae emergence, which help the parasite infect the rat hosts Due to the isolated nature of these sub-populations, adaptations in one group may make them more amenable to host infection than others, possibly explaining the significant differences from one study site to another.

 A. Théron, J. P. Pointier, S. Morand, D. Imbert- Establet and G. Borel Long-term dynamics of natural populations of Schistosoma mansoni among Rattus rattus in a patchy environment. Parasitology 104: